libdot: externs: pull Chrome externs from closure
libdot: externs: pull Chrome externs from closure

Rather than hand maintain these, download the ones from the closure
compiler project automatically based on the version we're using.

There's a few minor changes in hterm/nassh/terminal that we need to
adjust to the new baseline externs.

Change-Id: I93b81436f217fb92b813deceffdeb5c41bddd0c9
Reviewed-on: https://chromium-review.googlesource.com/c/apps/libapps/+/2160432
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Joel Hockey <9ecfc52dc2ffef73afac73f3495f294aeddf1231@chromium.org>

chromium/hterm,chromium/hterm,chromium/hterm,chromium/hterm